The past several decades have seen the development of communications systems that provide for the real-time distribution of information on a global scale. The development of such global communication systems has evolved along several paths that use either ground-based or satellite-based communication. Satellite-based systems have been employed for many years to distribute voice, data and video signals for global broadcasting of news and sporting events, for example. With the advent of newer high power satellite systems and technology, direct broadcast television systems are available that provide for broadcasting of television signals from up to 100 stations using a single satellite. Individuals install an antenna and a satellite receiver that receives the broadcasts directly by way of the satellite and display them on a television monitor.
In order to provide for more personal communication or teleconferencing between individuals, companies such as AT&T, for example, have developed a video telephone system that employs a small (3-4 inch) television monitor in combination with a conventional telephone. The video telephone system typically uses fiber optic links to provide a sufficient bandwidth to carry the video along with the voice signals. However, this type of system does not have enough bandwidth to provide for full motion video.
In order to provide a more cost-effective solution than terestial communication alternatives for low duty cycle, wide bandwidth applications, such as personal video, the assignee of the present invention has developed a high data rate satellite communication system that provides for the communication and distribution of full motion video, voice, and data signals, to provide for a wide array of data communications including personal teleconferencing between individuals. This system is disclosed in U.S. patent application Ser. No. 08/142,524, filed Oct. 21, 1993, entitled "High Data Rate Satellite Communication System", the contents of which are incorporated herein by reference. This high data rate satellite communication system comprises a plurality of very small user terminals (VSAT's) that are linked by and that communicate with each other by way of a satellite relay system. A network control center provides control signals that control the satellite relay system and coordinate linking of terminals to each other. The system employs frequency division multiplexing on uplinks from the terminals and the network control center to the satellite relay system. The system employs time division multiplexing on downlinks from the satellite relay system to the terminals and the network control center.
To provide for efficient bandwidth utilization in this high data rate satellite communication system and to increase the number of users that may be supported by the system, the assignee of the present invention has developed a new frequency reuse technique and data coding structure. This system is disclosed in U.S. Pat. No. 5,473,601, issued Dec. 5, 1995, entitled "Frequency Reuse Technique For A High Data Rate Satellite Communication System", the contents of which are incorporated herein by reference.
However, communication systems utilizing Kaband frequencies and multi-beam payloads with on-board demodulation and routing to achieve acceptable throughputs to low cost, very small user terminals must tolerate sizable variations in atmospheric attenuation due to rain and other climatic effects. For example, if there is rain fade on the downlink only, the receiving ground station would detect poor performance. In the current art, as applied to transponded (i.e., non-demodulation/remodulation) systems, the receiving ground station would then transmit feedback to the transmitting ground station identifying a need for increased power. The ground stations receiving the adjacent channels detect poor performance and request an increase in power of their corresponding transmit ground stations. For a transponded system, this increase in uplink power translates to an increase in downlink signal-to-noise ratio. However, in a system which utilizes on-board demodulation and remodulation and a plurality of simultaneously uplinking user terminals, an increase in uplink power causes interference with adjacent uplink channels causing adjacent channel degradation. Application of the current art to such a system would cause all uplinks to migrate toward maximum power and, thus, system runaway occurs. Thus, a modification in the current art of uplink power control is needed to handle systems with on-board demodulation and remodulation and a plurality of simultaneously uplinking users.
An alternative to the current art which might be used on non-demodulation and remodulation systems is to simply oversize the uplink powers. However, for a multi-user system this would cause increased uplink interference. In addition to increased uplink interference, oversizing the uplink power to accommodate rain fades increases signal power imbalances seen at the satellite demodulator. Signal level imbalance affects the necessary complexity of the on-board processor in terms of filter selectivity, A/D converter size, and computational precision. This increased complexity also increases the power required by the on-board processor.
Thus, there exists a need to control the uplink power so as to minimize the interference with other communication links within the system. There also exists a need to control the uplink power without adding complexity to the satellite.